π-Complexes of Tropolone and Its N-Derivatives : Ambidentate [O,O]/[N,O]/[N,N]-Cycloheptatrienyl Pentamethylcyclopentadienyl Ruthenium Sandwich Complexes

Tropolone and its N-derivatives isopropylaminotropone and diisopropylaminotroponimine react with [Cp*Ru(CH3CN)(3)]PF6 as the source of the electron-rich Cp*Ru+ synthon in a simple "capping reaction" to the first cyclopentadienyl/cycloheptatrienyl sandwich complexes containing two vicinal oxo or amino substituents. These heteroleptic, cationic C-5/C-7 pi-complexes, [Cp*Ru(C7H5)O2H](+) (4), [Cp*Ru(C7H5)O((NPr)-Pr-i)H](+) (5), and [Cp*Ru(C7H5)((NPr)-Pr-i)(2)H](+) (6), are air-stable 18-electron metallocenes without precedence. In solution, NMR spectroscopy proves the principal pi-coordination of the substituted eta(7)-cycloheptatrienyl ligands, whereas in the solid state a eta(5)-cycloheptatrienyl coordination mode with bent oxo/imino moieties prevails, as shown by X-ray single-crystal structure analyses. Chemically, these compounds are the conjugate Bronsted acids of neutral [O,O]-, [N,O]-, or [N,N]- metalloligands. Reaction with metal acetates or acetylacetonates by in situ deprotonation and complex formation gives access to trimetallic (M = Cu2+, 10-12) or tetrametallic (M = Fe3+, 13) complex cations. Their single-crystal structure analyses show square-planar or square-pyramidal (M = Cu2+) or octahedral (M = Fe3+) coordination motifs with peripheral, eta(5)-coordinated Cp*Ru moieties. Electrochemical studies on the trorucenes 4-6 showed a likely metal-based chemically reversible (4) or irreversible one-electron reduction (5, 6) as well as an irreversible one-electron oxidation for the N-substituted compounds 5 and 6. The behavior of the heterotri- and tetrametallic complexes 10, 11, and 13 was rationalized by a combination of cyclic and square-wave voltammetry as well as the combination of chronocoulometry and linear-sweep voltammetry and by comparison with the mononuclear copper or iron tropolone and topolonimine precursors 14-16. These studies indicate that in 10 and 11 the trorucene moieties are reduced first in two coincident or slightly separated one-electron reductions with the Cu2+ reduction at very negative potential, whereas in 13 the central Fe(III)-tris(troponolato) moiety is reduced prior to the trorucene appendices. An in vitro anticancer screening in MDA-MB-231 breast adenocarcinoma and HT-29 colon carcinoma cell lines showed enhanced antiproliferative activity for the [O,O]-coordinated iron and copper complexes.